(1) Field of the Invention
The present invention relates to traveling wave optical modulators and a method for the production thereof.
(2) Related Art Statement
JP-A 9-211,402 discloses that an optical waveguide substrate is bonded to an underlying fixing substrate in the state that an optical waveguide is opposed to the fixing substrate. At that time, a groove is preliminarily formed in the fixing substrate, so that the optical waveguide is exposed to air inside the groove. Then, the optical waveguide substrate is ground to a reduced thickness, thereby decreasing the effective refractive index of microwaves.
However, according to the technique described in JP-A 9-211,402, it was difficult to reduce the thickness of the substrate in actually producing a modulator from a manufacturing standpoint. If the thickness of the substrate is not more than 20 xcexcm, particularly not more than 10 xcexcm, there is the possibility that the substrate is cracked around the optical waveguide or that working strain remains in the substrate. Further, if the thickness of the substrate is reduced to not more than 20 xcexcm, an effect of confining a vertical component of light in the optical waveguide becomes stronger, so that the near field pattern of an optical waveguide is deformed flat. Therefore, mode shape mismatch increases between an optical waveguide and an external optical fiber, thereby leading to coupling loss. In this way, the prior art method has problems in reducing thickness of the substrate.
It is an object of the present invention to provide a traveling wave optical modulator for modulating a light propagating in an optical waveguide of an optical waveguide substrate made of a ferrodielectric electro-optic single crystal, which optical modulator effect high-speed modulation, prevent cracking and strain remaining in the substrate and also prevent deformation of the pattern of an optical waveguide mode. The present invention is also to provide a process for producing such a traveling wave optical modulator.
The traveling wave optical modulator according to the present invention comprises an optical waveguide substrate made of a ferrodielectric electro-optic single crystal and having a pair of opposed main planes, an optical waveguide formed on one of the main planes, at least one pair of electrodes provided on the other main plane of the optical waveguide substrate to apply a voltage for modulating a light propagating in the optical waveguide, a fixing substrate bonded to said one of the main planes of the optical waveguide substrate via an adhesive layer, said adhesive layer covering the optical waveguide being made of an adhering agent having a refractive index lower than that of the electro-optic single crystal.
A process for producing a traveling wave optical modulator comprising, (a) preparing a preform for an optical waveguide substrate, said preform being made of a ferrodielectric electro-optic single crystal and having a pair of opposed main planes, (b) forming an optical waveguide on one of the main planes, (c) bonding a fixing substrate to said one of the main planes of the optical waveguide substrate via an adhesive layer, adhesive being made of an adhering agent having a refractive index lower than that of the electro-optic single crystal and covering the optical waveguide at this time, (d) working the other main plane of the substrate preform to reduce its thickness for the formation of an optical waveguide substrate, and (e) forming at least one pair of electrodes on the other main plane of the optical waveguide substrate to apply a voltage for modulating a light propagating in the optical waveguide.
In the present invention, the following are preferable embodiments of the present invention.
(1) The traveling wave optical modulator is adapted to propagate a TE mode light through the optical waveguide.
(2) The thickness of the optical waveguide substrate is not more than 20 xcexcm.
(3) The fixing substrate is made of a material having a dielectric constant and a dielectric loss smaller than those of the electro-optic single crystal, respectively.
(4) The thickness of the adhesive layer is not less than 20 xcexcm, and the fixing substrate is made of a material having a dielectric constant and a dielectric loss not smaller than those of the electro-optic single crystal, respectively.
(5) The electro-optic single crystal is one or more single crystals from the group consisting of a lithium niobate single crystal, a lithium tantalate single crystal and a lithium niobate-lithium tantalate solid solution single crystal.
The following is a preferable embodiment of the process for the production of the traveling wave optical modulator. That is, the thickness of the optical waveguide is not more than 20 xcexcm.
These and other objects, features and advantages of the present invention will be appreciated reading the following description of the invention when taken in conjunction the attached drawings, with the understanding that some modifications, variations and changes could be made by the skilled person in the art to which the invention pertains.